If you keep your computer on all the time, volunteer its spare cycles to science with the Berkeley Open Infrastructure for Network Computing project.

BOINC includes the well-known SETI@Home project, which uses the idle time of millions of volunteer personal computers to crunch data from big radio telescopes to find extraterrestrial life. Along the same lines of SETI@Home, use your computer's idle time to "cure diseases, study global warming, discover pulsars, and do many other types of scientific research" with any of the other BOINC projects. Download the BOINC screensaver for your Windows PC and choose the projects you want to contribute to, and the percentage of time you want to donate to each. — Gina Trapani